1. The Field of the Invention
The present invention relates to a method for fabricating an interlayer dielectric film for a semiconductor device. Particularly, it relates to a method for fabricating an interlayer dielectric film for a semiconductor device capable of 1) overcoming functional limitations on borophosphosilicate concentrations in the film and 2) reducing erosion due to H.sub.2 SO.sub.4 boiling. This is accomplished by performing a surface treatment on a borophosphosilicate glass film after the deposition thereof onto a semiconductor substrate.
2. Description of Prior Art
Some very large scale integration (VLSI) processes would benefit from being performed at lower temperatures than those necessary for phosphosilicate glass (PSG) reflow (1,000.degree.-1,100.degree. C.) because such high temperatures result in excessive diffusion of junctions. Furthermore, metal oxide semiconductor gate oxides cannot be exposed to high temperature processing (i.e. beyond 900.degree. C.). However, flowable glass is still very desirable for facilitating film coverage over abrupt steps in the substrate topography. Glass flow temperatures as low as 700.degree. C. can be obtained by adding boron dopant (e.g. B.sub.2 H.sub.6) to the PSG gas flow to form the ternary (three component) oxide system B.sub.2 O.sub.3 --P.sub.2 O.sub.5 --SiO.sub.2, borophosphosilicate glass (BPSG).
BPSG flow depends on film composition, flow temperature, flow time, and flow ambient atmosphere. Reportedly, an increase in boron concentration of 1 wt % in BPSG decreases the required flow temperature by about 40.degree. C. However, increasing the P concentration beyond about 5 wt % does not further decrease BPSG flow temperature. Furthermore, there is an upper limit on boron concentration imposed by film stability. That is, BPSG films containing over 5 wt % boron tend to be very hygroscopic and unstable, and if used, should be flowed immediately following deposition. It has also been reported that rapid thermal annealing for 30 seconds at a temperature 100.degree.-175.degree. C. higher than that used in a conventional furnace step will result in equivalent BPSG flow. The ambient gas of the flow cycle also affects the flow mechanism. By using a steam ambient instead of N.sub.2, the minimum required flow temperature is reduced by about 70.degree. C.
In addition to exhibiting these low temperature flow properties, BPSG (like PSG) is an alkali ion getter and exhibits low stress. Because of its doping, however, BPSG can also be an unwanted diffusion source to underlying silicon. It is found that BPSG is primarily a source of phosphorus, and phosphorus outdiffusion is increased at higher boron concentrations.
Because of the BPSG characteristics mentioned above, atmospheric pressure chemical vapor deposition (APCVD) using O.sub.3 and organic-source has been recently used. This example is disclosed at page 6 of the Ozone/Organic-Source APCVD for ULSI Reflow Glass Films, by Yasuo IKEDA, Youichirou NUMASAWA and Mitsuru SAKAMOTO, VLSI Development Division, NEC Res. & Develop. No. 94, published in July, 1987.
When BPSG used as an interlayer dielectric film becomes submicron, BPSG reflow causes junction breakdown due to thermal stress, so low temperature reflow processing is desirable. According to conventional process technology, planarization of interlayer dielectrics (ILD) is accomplished by carrying out H.sub.2 SO.sub.4 boiling, then BPSG reflow for deposition. FIGS. 2A and 2B show processing flow diagrams according to a conventional art technology. FIG. 2A depicts a pattern where a conductor 1 is formed and FIG. 2B depicts that BPSG 2 is deposited on the conductor 1. After BPSG with high concentrations of B and P (i.e., from 4 to 15 wt % of both B and P) is deposited, H.sub.2 SO.sub.4 boiling is carried out together with a surface treatment, and BPSG is then flowed. As the manufacture of from 1M bit DRAMs to 64M bit DRAMs has been achieved, BPSG low temperature reflow under 850.degree. C. is required.
The outer-shell electron configuration of B or P in BPSG is different from that of silicon ions. As the number of outer-shell electrons for B and P is respectively changed from 3 to 4 and from 5 to 4, charge deficiency or increase in the oxygen ion occurs. As the network structure between atoms becomes weak, the melting point drops. As the concentration of B and P in the film increase, the melting point drops further because the network structure becomes weaker. However, in a case where BPSG films containing B and P in high concentrations are deposited, then are H.sub.2 SO.sub.4 boiled without surface treatment, the films exhibit a significant tendency to crack. If BPSG films are left exposed in the air, they react with atmospheric humidity and crack. Thus, an expected benefit in using BPSG cannot be realized.